In an AC power system, power transformers are used to convert electric power from one potential level to another.
Transformers are one of the most essential elements of an electric power system. They are widely used in electric networks because the generation, transmission, and distribution of power require different voltage levels. For example, in a large power utility, the number of transformers can exceed 1000. They link electric loads to power supplies through an interconnected power network and satisfy the requirements of both parts. The use of transformers helps to reduce losses in an AC power system.
Transformers are one of the most expensive pieces of equipment in a power grid. A large power transformer in a 500 kV system may cost in the order of a million dollars. In 1996, the total sale of power transformers in the United States was over US$ 11 million. In terms of numbers, more than 8000 transformers, of at least 2 MVA and up to more than 100 MVA, were sold in the same year all over the world.
Since transformers play a vital role in the operation of AC or HVDC electric networks, it is important to ensure that they are operating efficiently and reliably. Furthermore, the failure of a transformer can be potentially devastating to personnel safety and to the environment. Transformers may fail explosively causing great personal injuries to people around them and damage to the surrounding equipment. The environment can be adversely affected by the leakage of oil. A failure also has a large economic impact due to its high cost of replacement and repair and lost revenue while it is out of service. During its outage, the customers of a utility can be greatly inconvenienced, which could result in a loss of goodwill towards the utility.
Transformers are well known in the art, and can include dry type transformers and fluid-filled transformers, with different core configurations. FIG. 1 illustrates an iron-core transformer which is typically encased within a tank. Generally, the iron-core transformer consists of one or more sets of windings that are coupled through a common magnetic core. The main components of transformer 10 shown in FIG. 1 include a laminated iron core 12, and a set of windings 14a and 14b, where winding 14a can be a high voltage winding while winding 14b can be a low voltage winding. Each set of windings normally consist of two or three windings: a primary winding, a secondary winding, and sometimes a tertiary winding, mounted concentrically on top of each other. As shown in FIG. 1, windings 14a and 14b are wrapped around the legs of the rectangular window iron core 12. The windings 14 are usually made of copper or aluminum, and can be shaped as either wires or sheets with an insulating layer 16 separating successive layers of the windings. The windings 14 are clamped to the tank structure to maintain their physical positioning. Core clamps 18 are used to fix the core to the tank structure. Ceramic bushings are used to isolate the windings from grounded structures of the transformer such as the oil tank. Mineral oil is typically used as insulation medium, and for cooling the transformer. In operation, a time-varying flux created by one winding induces a voltage in the other winding. FIG. 1 represents one known configuration of a core-type transformer, and those of skill in the art will understand that there are a number of known configurations.
Inevitably in service transformer failures occur due to either internal failures and/or external failures. As the context of the present invention is related to internal failures, external failures will not be discussed.
Internal failures are faults that occur inside the tank, such as short circuits between windings, short circuits between windings and iron, short circuits between turns, insulation deterioration, loss of winding clamping, partial discharges and winding resonance. One of the primary causes of internal failures is winding movement, or displacement, which can lead to insulation deterioration, and winding collapse. When the insulation deteriorates, windings can electrically contact each other, resulting in high current flow, partial discharges, and very severe and expensive faults.
An external short circuit fault, for example, created by lightning strike or ground fault, is the most likely factor to cause winding movement. Assuming a copper winding is wound on a ferromagnetic core and the transformer is in service, the currents carried by the windings will produce the predominant flux in the axial direction. The interaction of the current in the coils in the circumferential direction and the axial flux field will therefore produce a radial outward electromagnetic force. FIG. 2 illustrates possible movement of a winding 20 relative to a core 22 of a typical iron-core transformer. A current 1 flowing through the winding 20 in the direction as indicated by the arrows, can suffer from movement in the axial direction as shown by arrows 24, and outward winding movement in the radial direction as shown by arrows 26. There is, however, a leakage flux around individual turns and near the ends of the windings which has a component in the radial direction. This radial flux induces an axial inward electromagnetic force. Under normal operation of power transformer, the windings are designed to withstand the mechanical pressure described above. However, when short circuits happen, the electromagnetic forces induced in the windings are increased dramatically and threaten the insulation layers severely. For example, if a transformer's leakage impedance is 10%, its short circuit current will be 10 times the rated current and the mechanical stress will be roughly 100 times of the normal stress under the rated load current.
In the event of a short circuit situation, the winding will stretch out in the radial direction and compress in the axial direction. The radial forces in a 10 MVA transformer can exceed 100,000 lbs. Such huge electromagnetic forces will inevitably loosen the winding clamp structure, which is the main mechanical support of the winding and cause the distortion or movement of the winding.
Inrush currents, due to the powering up of the transformer circuit, and vibration forces increase the electro-magnetic forces in the same way as short circuits. The frequent fluctuation in generation or load will also put burdens to the electrical and mechanical strength of the windings, accelerate the loosening of the winding clamps, and eventually cause the winding to move or even break down. Furthermore, winding bulges or sharp edges of the coil can cut through insulation and cause short circuits between turns, which can be exacerbated as the winding moves.
Since a large number of failures are due to the windings, techniques for diagnosing the health of a transformer have been proposed, since it is useful to be able to assess the health of a winding to enable prediction of remaining life, capacity limits, and preventive maintenance. A short summary of some of these methods follows.
There are several known methods to assess the health of a winding, including detection of insulation degradation. By way of example, insulation degradation in transformers can be monitored by methods that detect partial discharges. Partial discharge detection methods include Dissolved Gas Analysis and Tan-Delta techniques. It is notable that Dissolved Gas Analysis methods detect the chemical reaction products of insulation degradation, and therefore partial discharge must have occurred before these techniques are useful. The Tan-Delta similarly detects damage that has already occurred.
The winding ratio test measures the numbers of turns of both primary and secondary windings and calculates the ratio between them. By comparing the measured winding ratio with the ratio of rated primary and secondary voltages as shown on the nameplate of the transformer, shorted turns or open winding faults may be detected. However, an outage and isolation of the transformer is required for the purpose of measurements.
The winding resistance test, which is similar to the winding ratio test, except that it measures the winding resistance rather than the number of winding turns. Additionally, a very precise ohmmeter is needed, which will assure the accuracy of a fraction of an ohm. The measured resistance will be compared with the previous measurement referred to the same temperature. Measurements are conducted for different phases and different tap-changer positions. This method detects the condition of the winding conductor directly. However, it requires a transformer outage and is usually performed in the factory or a laboratory.
The 60 Hz transformer impedance test measures the input voltage, input current and input power while shorting the low voltage winding of the transformer. The before and after results are then compared with the before and after short circuit. This test is insensitive to small winding movements.
The leakage reactance measurement (LRM), which can be achieved with the same test set-up as the short circuit impedance measurement, is based on the increased leakage reactance resulting from the radial outward force on the outer winding and the radial inward force on the inner winding induced by short circuits.
Another way to assess the health of a winding is to detect winding displacement. Detection of winding displacement is advantageous because such methods permit detection of deterioration prior to actual winding damage. Known winding displacement detection methods include detection of increases of the audible noise, visual inspection, short-circuit impedance measurement, vibration analysis, low voltage impulse, and swept Frequency Response Analysis (‘FRA’) techniques.
The FRA technique compares the input admittance function Y(ω) of the displaced winding with the Y(ω) of an equivalent healthy winding. This technique is well known in the art, and looks at the transformer as a lumped impedance and measures the admittance function Y(ω) as the ratio of input current to applied voltage for a range of high frequencies (e.g., 1 to 10 MHz). The idea of the FRA test is to compare the “signature” of the transformer, as defined by the shape of the Y(ω) function, as it changes with aging.
FIG. 3 shows measurement plots made using the FRA technique of a healthy winding 20 versus that of a displaced winding 22. The admittance Y(ω) in siemens is plotted on the vertical axis while frequency in MHz is plotted on the horizontal axis. Two main problems exist with this technique. First, the frequency response differences between the two curves only begin to appear at frequencies beyond 2-4 MHz. Unfortunately, online measurements are corrupted due to electrical noise interference, which becomes problematic at these frequencies. Second, the observed response distortion is difficult to directly correlate any specific problem with the displaced winding, and in particular, to the amount of winding displacement.
The main problem with the FRA technique is that the Y(ω) function is highly oscillatory because it includes all the winding electrical parameters: resistance, inductance, and capacitance and, in addition, is very sensitive to electrical noise and to numerical noise in the processing of the signals. As a result, FRA tests are normally performed off-line with the equipment removed from service and in a very controlled test environment. Taking a power transformer out of service is very expensive and sometimes not even feasible because of service continuity constraints.
It is, therefore, desirable to provide a method and system for assessing the health of electrical windings in transformers without having to remove the transformer unit from service, while providing effective determination of winding displacement and/or faults in the winding.